Just because they get a good sound doesn't mean they need to be playing that instrument. The flute is a hard instrument to get the fingers down and if you have a student on there that it just doesn't 'click' then you will have issues. Flutes are generally slower than the others, playing wise, but they should at least have Bb-G down by now along with a couple of short songs and several pages of the beginner book. I don't understand why people think anyone can just pick up a flute and play just because they get a good sound. Don't set the students up for failure because you don't know how to put them on an instrument.